In an injection control system of internal combustion engine, for example, as described in Patent Literature 1 (JP2012-102657A), by increasing a target drive current of the fuel injection valve as a target fuel pressure becomes higher, even when the fuel pressure is high, the fuel injection valve can be reliably opened.
The inventor has examined a system used in an internal combustion engine of a cylinder-injection type. As illustrated in FIG. 5, in order to prevent a drive current of the fuel injection valve from frequently changing with a change of a fuel pressure in setting of the drive current according to the fuel pressure, the system uses a drive current profile having a hysteresis in change characteristics of the drive current with respect to the fuel pressure. In this case, the drive current is, for example, a peak current.
However, when the drive current is set using the drive current profile having a hysteresis, the drive current may vary even at the same fuel pressure, causing a variation in an injection amount. Some internal combustion engines of a cylinder-injection type execute intake stroke injection that injects fuel in an intake stroke and a compression stroke injection that injects fuel in a compression stroke during execution of catalyst early-stage warming control that early warms up a catalyst purifying an exhaust gas, and a particulate matter (PM) emission greatly varies according to the injection amount of the compression stroke injection. For this reason, when the injection amount of the compression stroke injection largely varies due to the hysteresis in the drive current profile, the PM emission may exceed an allowable upper limit, or the injection amount is put into an accidental fire region. Such phenomenon can occur in an expansion stroke injection that injects fuel in an expansion stroke.